1. Field of the Invention
This invention relates to a method of measuring the level of an after-glow of a stimulable phosphor sheet in a radiation image read-out method for reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and detecting the emitted light. This invention also relates to a method of adjusting radiation image read-out conditions to optimal values in the radiation image read-out method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to a stimulating ray beam such as a laser beam which causes the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, and the radiation image of the object is reproduced as a visible image by use of the image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having a desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image recording and reproducing system using a stimulable phosphor sheet, compensation for deviation of the level of the radiation energy stored on the stimulable phosphor sheet from a desired level can easily be carried out by adjusting the read-out gain to an appropriate value when photoelectrically reading out the light emitted by the stimulable phosphor sheet upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a fluctuation in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, variation in the sensitivity of the stimulable phosphor sheet or the photodetector, changes in radiation dose resulting from differences in the condition of the object, and the like. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low. Further, it is possible to obtain a radiation image having a high image quality of high contrast, high sharpness and low noise, and the like, by converting the light emitted by the stimulable phosphor sheet into an electric signal, and processing the electric signal as desired.
However, in order to eliminate various influences caused by variations in radiographic exposure conditions and/or to obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is necessary to ascertain such image input conditions of the radiation image stored on the stimulable phosphor sheet as, for example, the level of radiation dose used for image recording, or the image input pattern which is determined by the portion of the object (e.g. the chest or the abdomen of the human body) or the image recording method used, such as plain image recording or contrasted image recording, before reproducing the radiation image to a visible image, and then to adjust the read-out gain to an appropriate value or to process the electric signal appropriately based on the ascertained image input conditions or the image input pattern. The image input conditions and the image input pattern will hereinafter be simply referred to as the image input information when they are referred to generically. It is also necessary to adjust the scale factor to optimize the resolution in accordance with the contrast of the image input pattern, and to adjust image processing conditions to appropriate values in the case where image processing such as gradation processing is carried out on the read-out image signal.
Ascertaining of the image input information may be carried out prior to the visible image reproduction by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed method, a read-out operation for ascertaining the image input information of a radiation image stored on a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is carried out in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out is carried out. In the final read-out, the read-out gain and/or the scale factor is adjusted to an appropriate value, and/or an appropriate image processing is conducted, on the basis of the image input information obtained by the preliminary read-out.
As mentioned above, the level of stimulation energy of the stimulating rays used in the preliminary read-out is lower than the level of stimulation energy of the stimulating rays used in the final read-out. Specifically, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out.
Various methods may be used for approximately ascertaining the image input information on the stimulable phosphor sheet from preliminary read-out image signals obtained by the aforesaid preliminary read-out. One method is to utilize a histogram of the preliminary read-out image signals. Specifically, the image input information can be ascertained from, for example, a characteristic value such as the maximum signal value, the minimum signal value, or a signal value at a point where the frequency is the maximum in the histogram. Therefore, a visible radiation image can be reproduced by adjusting the read-out conditions such as the read-out gain and the scale factor (latitude) and/or the image processing conditions on the basis of the characteristic value.
However, the read-out conditions for the final read-out and/or the image processing conditions adjusted in the manner as mentioned above are often adversely affected by an instantaneous light emission after-glow emanated from the stimulable phosphor sheet, and become inappropriate for the actual image input information on the stimulable phosphor sheet. By "instantaneous light emission after-glow" is meant the after-glow of light instantaneously emitted by a stimulable phosphor sheet when the sheet is exposed to a radiation to have a radiation image stored thereon, the after-glow continuing to be emitted by the sheet for a considerable period of time, for example, for a period within the range of several seconds to several tens of seconds, after the exposure of the sheet to the radiation is ceased. Therefore, signal components caused by the instantaneous light emission after-glow are naturally contained in the preliminary read-out image signals, and the levels of the preliminary read-out image signals become higher as a whole than the signal components representing the actual image information. As a result, the histogram of the preliminary read-out image signals shifts to the large signal value side (i.e. large light emission amount side), and does not correctly reflect the actual image input information. That is, the histogram becomes as if it were created for an image having a density higher as a whole than the density of the image actually stored on the stimulable phosphor sheet.
Therefore, if the radiation image stored on the stimulable phosphor sheet is read out by use of the read-out conditions or the like adjusted based on such a histogram and a visible image is reproduced by use of the image signals detected in this manner, image density may fail to be formed at the low density region of the reproduced visible image.
Besides the adverse effects on the read-out conditions for the final read-out and/or the image processing conditions as mentioned above, the instantaneous light emission after-glow is detected also in the course of the final read-out, and noise components caused by the instantaneous light emission after-glow are contained in the final read-out image signals, thereby adversely affecting the image quality of the reproduced radiation image.
In order to eliminate the aforesaid problems, it has been proposed in, for example, Japanese Patent Application No. 58(1983)-108238, corresponding to U.S. Pat. No. 4,571,493 to subtract signal components caused by the instantaneous light emission after-glow from the image signals, and to process the image signals obtained by the subtraction as signals representing the actual radiation image stored on a stimulable phosphor sheet. For carrying out such subtraction processing, it is first necessary to accurately measure the level of the instantaneous light emission after-glow. For this purpose, as disclosed also in the aforesaid Japanese Patent Application No. 58(1983)-108238, corresponding to U.S. Pat. No. 4,571,493 the level of the instantaneous light emission after-glow has heretofore been carried out by sampling the output of a light detection means when stimulating rays for scanning the stimulable phosphor sheet have finished scanning along a single scanning line in a main scanning direction and are not then impinging upon the stimulable phosphor sheet.
However, with the aforesaid method of measuring the level of the instantaneous light emission after-glow, a signal representing the radiation image of the object and a signal representing the level of the instantaneous light emission after-glow are alternately generated by the light detection means. Therefore, complicated processing is necessary for separation of the signal representing the radiation image of the object and the signal representing the level of the instantaneous light emission after-glow from each other, or the like.